Polarization mode dispersion (PMD) of an optical fiber is a difference in the group delay between two basic waveguide modes of the optical fiber. The PMD occurs due to anisotropy of the optical properties of an optical fiber. The causes of the anisotropy of optical properties include external factors such as a lateral pressure, a bending, or a twist of an optical fiber other than internal factors, such as the structure or composition of an optical fiber, or the anisotropy of internal stress. Since the PMD is a factor that limits the transmission capacity of an optical fiber, various technologies for reducing the PMD of an optical fiber have been developed.
Patent Document 1 discloses a method, in which the PMD of an optical fiber is reduced by imparting a twist to the optical fiber. In this method, in the step of drawing an optical fiber from a glass preform, a device for twisting the optical fiber is disposed between a device for pulling the optical fiber and a device for winding up the optical fiber onto a reel, thereby imparting a twist to the optical fiber and winding up this twisted optical fiber onto the reel. Alternatively, in the step of rewinding an optical fiber, which is wound on a reel, onto a different reel, a twist is imparted to the optical fiber by winding up the optical fiber while twisting the same. By imparting an appropriate magnitude of twist to an optical fiber, the PMD of the optical fiber can be reduced. For example, it is shown here that the PMD can be reduced to one-fifth or less in any optical fiber having a beat length in a range of 5 to 50 m by imparting a twist of at least once/m to the optical fiber.
Note that, in Patent Document 1, a torsion with stress is defined as a twist and a torsion without stress is defined as a spin. The same definitions as these are also used in this specification.
Patent Document 2 discloses a method imparting, in the step of drawing an optical fiber from a glass preform, a spin to the optical fiber by drawing an optical fiber from the preform while twisting the same. According to this document, the PMD may be reduced particularly well when a spin, whose polarity is reversed at a cycle varying at 1 m or more and in the longitudinal direction, is imparted to an optical fiber having a beat length longer than 0.5 m.
In Non-Patent Document 1, the behavior of PMD due to external factors in an optical fiber imparted with a spin is described. According to this document, in an optical fiber without spin, the PMD differs depending on the direction of a lateral pressure, while in the optical fiber with a spin, the PMD is averaged with respect to the direction and thus no longer depends on the direction of a lateral pressure, but as the lateral pressure increases, the PMD may similarly increase with or without spin.    Patent Document 1: US Patent Application Publication No. 2006/0133751    Patent Document 2: U.S. Pat. No. 6,993,229    Non-Patent Document 1: M. J. Li et al., Optics Letters, vol. 24, no. 19, pp. 1325-1327 (1999)    Non-Patent Document 2: C. D. Poole, et al., Optics Letters vol. 16, pp. 372-374 (1991)    Non-Patent document 3: J. Noda et al., J. Lightwave Technol. v. 4, pp. 1071-1089 (1986)    Non-Patent document 4: R. E. Shuh et al., Electronics Letters, vol. 31, no. 20, pp. 1772-1773, (1995)